Phellem is a type of tissue found in the outer layer of plant stems and roots. It is commonly known as cork and is produced by the cork cambium, a layer of cells that divides to form new cells. Phellem serves as a protective barrier, helping to prevent water loss and protect against pests and diseases. This tissue is important for the overall health of the plant. In many trees, such as the oak, phellem can accumulate over time, leading to the formation of thick, insulating layers of cork. This adaptation allows plants to survive in various environments by providing durability and resilience.